1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and a method for transmitting an uplink signal in a mobile communication system using an Orthogonal Frequency Division Multiple Access (OFDMA) scheme.
2. Description of the Related Art
In a 4th generation (4G) communication system, which is the next generation communication system, there is ongoing research to provide users with services having various Qualities of Service (‘QoS’) and supporting a high speed transmission.
As a scheme useful for high speed data transmission in wire or wireless channels, an Orthogonal Frequency Division Multiplexing (OFDM) scheme is now being actively researched. The OFDM scheme, which transmits data using multiple carriers, is a special type of a Multiple Carrier Modulation (MCM) scheme in which a serial symbol sequence is converted into parallel symbol sequences and the parallel symbol sequences are modulated with a plurality of mutually orthogonal sub-carriers before being transmitted.
In order to provide a high-speed and high-quality data service to a Mobile Station (MS), a Base Station (BS) must select an optimum modulation and encoding scheme based on a wireless channel environment. The wireless channel environment is determined by a number of factors, such as white noise, frequency selective fading, shadowing, Doppler effect due to movement of the MS, Inter-Symbol Interference (ISI) due to delay spread, etc. Therefore, the BS must select a proper Modulation and Coding Scheme (MCS) in response to the wireless channel environment changing due to the factors described above. To this end, an exact channel estimation is required to precede such a selection. Understandably, it goes without saying that the MS also can perform the channel estimation.
A pilot signal is used for the channel estimation. Conventionally, a receiver performs channel estimation by interpolating a pilot signal transmitted from a transmitter. Therefore, the more pilot signals received during a unit time interval, the more exactly the receiver can estimate a channel.
On an assumption that the transmitter is a BS and the receiver is an MS in the OFDMA mobile communication system, the BS can transmit a plurality of pilot signals' to the MS in the entire frequency bands, and the MS can exactly estimate a downlink channel by using the plurality of received pilot signals. However, when the transmitter is an MS and the receiver is a BS, the MS transmits a pilot signal in a frequency band that does not overlap that of another MS, and the BS estimates an uplink channel by using the received pilot signal from the MS. Therefore, the number of uplink pilot signals is inevitably smaller than or equal to that of the downlink pilot signals, which degrades the performance throughput of the uplink channel estimation. If the number of the uplink pilot signals is increased in order to solve this problem, just as many data signals are inevitably reduced, thereby decreasing the entire throughput.